In /proc/meminfo, does Dirty include or exclude Writeback?

Question

Dirty %lu - Memory which is waiting to get written back to the disk.

Writeback %lu - Memory which is actively being written back to the disk.

-- man proc

Does Dirty include Writeback? Or is it excluded?

My kernel version is 4.18.16-200.fc28.x86_64.

Answer 1

“Dirty” and “Writeback” are separate stats and page states; for proof of this, see for example node_dirty_ok:

nr_pages += node_page_state(pgdat, NR_FILE_DIRTY);
nr_pages += node_page_state(pgdat, NR_UNSTABLE_NFS);
nr_pages += node_page_state(pgdat, NR_WRITEBACK);

return nr_pages <= limit;

If one included the other, that would be taken into account here.

/proc/meminfo doesn’t process the corresponding values either:

show_val_kb(m, "Dirty:          ",
        global_node_page_state(NR_FILE_DIRTY));
show_val_kb(m, "Writeback:      ",
        global_node_page_state(NR_WRITEBACK));

Answer 2

I think Dirty excludes Writeback.

dd if=/dev/zero of=~/X.img bs=1M count=1 ; sync & for i in 1 2 3; do grep -E '^(Dirty:|Writeback:|MemFree:|Cached:)' /proc/meminfo ; done
1+0 records in
1+0 records out
1048576 bytes (1.0 MB, 1.0 MiB) copied, 0.00316564 s, 331 MB/s
[1] 13140
MemFree:         1461832 kB
Cached:          1475188 kB
Dirty:               132 kB
Writeback:          1032 kB
...

(Although this is only proof if /proc/meminfo is guaranteed 100% consistent).